The STROBE statement for cross-sectional studies was adopted (Moreno-Ramirez et al. 2014; Vandenbroucke et al. 2014; Bolignano et al. 2013). The principles of the Italian data protection (196/2003) were guaranteed.
In consideration of the recent scientific evidence which highlighted the effectiveness of the ST (Dwyer and Davis 2013) and PUT (Youdas et al. 2010) to exhaustion, in the present study, the latter executions were considered as measures of reference to establish an eventual SUT to exhaustion inter-relation for the evaluation of muscle endurance. Therefore, to achieve the aim of this study and to verify the experimental hypothesis, the individual outcomes of the SUT execution to exhaustion were compared to those of both ST and PUT.
Although these tests refer to different movements, and muscle group contractions, the PUT and ST trials performed till exhaustion were already considered for the muscle endurance evaluation in previous studies (Dwyer and Davis 2013; Chulvi-Medrano et al. 2012). For the same reason, it seems conceivable that SUT trails performed till exhaustion are linked to a reliable (and coherent to PUT and ST) muscle endurance evaluation. Furthermore, according to literature (Cohen 1992), the percentile discrimination could be applied to identify the specific grades of muscular endurance, and guarantee useful normative values for future studies. In particular, the ranges between the 1st and 25th (i.e., 25th), 25th and 75th (i.e., 50th), and 75th and 100th (i.e., 75th) percentile were able to classify the muscular endurance as low, average, and high level, respectively.
All participants full filled a proper questionnaire containing two main sections: (1) information about the study design; (2) information about subjects’ health status and daily activities. Three-hundred-eighty-one participants volunteered for the study (28.5 ± 10.0 years; 168.2 ± 8.9 cm; 65.1 ± 11.1 kg). These where 194 male (27.5 ± 10.2 years; 173.6 ± 7.0 cm; 71.2 ± 5.2 kg), and 187 females (29.6 ± 10.1 years; 162.6 ± 7.1 cm; 58.7 ± 8.9 kg). The participants were recruited in the study according to the following inclusion criteria: (1) The obtaining of a medical examination that certified a good state of health; (2) were not regularly taking any medications (regular intake was defined as at least once a week); (3) were not during the first two days of the menstrual cycle; (4) were not under diet restriction regimen; (5) were beginners with sport and fitness activities.
Each subject had to perform the following tests to exhaustion: SUT, PUT, and ST. Each test has been performed in a different day, within a period of 7 days. The random sequence was balanced as follow: each participant was randomly assigned to one of the three groups we created (Group A: ST, PUT, SUT; Group B: PUT, ST, SUT; Group C: SUT, ST, PUT). Two interval days were planned for each group A, B, C. The same operator administered each test, supervising the correct execution, and inviting participants to perform till exhaustion. In particular, the PUT could be considered correctly executed if the following criteria are guaranteed: The exercise was performed on a flat, stable surface, hands placed slightly wider than shoulder-width apart, and fingers pointed forward. Participants were instructed to maintain a neutral spine and feet together position throughout the entire movement. Once again, in order for the repetition to be recorded the correct depth needed to be met. Participants were instructed to lower the body until the chest was within 2 inch from the floor (Snarr and Esco 2013). The ST execution referrers to: place feet a little wider than shoulder-width apart. Extend the arms out straight. Initiate movement by inhaling and unlocking the hips, slightly bringing them back. Keep sending hips backward as the knees begin to flex. Bring hips parallel to ground. Return to standing position (Swinton et al. 2012). The correct SUT execution is provided by: The subject lay supine on the floor with 90° flexion in the knee joints, hands at the side of their head, and with elbows pointing straight forward. To do a correct sit-up execution the elbows should touch the knees and then go back so the shoulders touch the floor (Blomqvist et al. 2013). The number of repetitions performed was recorded to quantify the grade of endurance in each test. The condition of exhaustion (i.e., execution till failure) consisted to the inability to perform another repetition. In all tests, the participants were asked to constantly exercise through the concentric and eccentric phase of movements without any rest between the upper and lower grade of movements. The cadence of movements was controlled by a beep sounds coming from the Apps Runtastic Fitness© GmbH 2015.
Firstly, the percentile distribution (i.e., 1st–25th, 25th; 25th–75th, 50th; 75th–100th, 75th) related to the number of repetitions performed during the SUT, PUT, and ST to exhaustion were calculated to provide the upper limit for low limit, average, and lower limit for high muscular endurance, respectively. Afterwards, to verify the grade of inter-relation of the three observed tests (i.e., SUT, PUT, ST), a linear regression analysis was adopted using the software STATISTICA v.8.0 (StatSoft©, Tulsa, USA). In particular, the SUT was considered as dependent variable. Statistical significance was set at an alpha level of p < 0.05. The Bland–Altman plots were obtained through the GraphPad Prism 5 for Windows (San Diego, CA, USA).